Nozzle

ABSTRACT

A nozzle ( 1 ) for use in coating a web-like material by means of high-pressure spraying techniques is manufactured by forming a piece having a tapered duct ending in a closed tip, with a transverse V-shaped groove ( 3 ) subsequently machined in the tip. The angle of the V-shaped groove ( 3 ) is in the range from 25 to 50°, such as 35 to 45°.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national stage application of internationalapp. No. PCT/FI2003/000702, filed Sep. 26, 2003, and claims priority onFinnish App. No. 20021719, filed Sep. 26, 2002.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The invention relates to the coating of a moving web-like material usinghigh-pressure techniques and it concerns the nozzle used in suchcoating. The invention can be used especially in paper coating.

In paper coating, a coating composition is applied to the paper surfacewith a special view to enhancing the printing characteristics of paper.Conventionally, presses, knife applicators and film-transfer deviceshave been used for coating. These techniques are difficult to implementreliably, especially when an increase in the running speed or coating ofvery thin paper is required.

Spray coating has appeared as the most recent coating technique. It hasthe special advantage of not requiring any mechanical coating means,such as an abrasive knife or rotating rod, in contact with the web.High-pressure spray techniques have proved particularly promising. Herethe coating composition alone, without any gaseous medium, is drivenunder high pressure through a nozzle with small orifices, thecomposition being diffused (atomized) into small droplets. The pressuremay be e.g. in the range from 1 to 200 MPa and the nozzle orifice areae.g. in the range from 0.02 to 0.5 mm². A typical maximum droplet sizeis approximately 100 μm. Such an apparatus comprises a nozzle arrayhaving one or more nozzle rows transverse to the path and consisting ofa plurality of nozzles. The nozzles are disposed so as to cover the webas evenly as possible with the jets. Then jets formed by adjacentnozzles in a nozzle row overlap appropriately at their edges. The jetshape provided by the nozzle depends on the shape of the nozzle orifice.The usual aim is a fan-shaped jet, which is larger in the transversedirection than in the longitudinal direction of the web. Then the nozzleorifice is accordingly oval. To achieve regular coating, the fans arepreferably disposed obliquely to the direction of travel of the web.

Spray coating of paper is described e.g. in the papers FI-B-108061(corresponding to WO 9713036) and Nissinen V, OptiSpray, the New LowImpact Paper Coating Technology, OptiSpray Coating and SizingConference, Finland, Mar. 15, 2001.

Nozzles can be manufactured by making a piece of a suitable material,e.g. a highly wear-resistant material, the piece having a tapered ductending in a closed tip, the desired nozzle orifice being subsequentlymachined in the tip. An oval orifice is provided if a transverseV-shaped groove is machined in the tip. The nozzle material may be e.g.a highly wear-resistant tungsten carbide composition (such as WC+Co).

SUMMARY OF THE INVENTION

The nozzle of this invention is for use in the coating of web-likematerial. The nozzle is made by machining in the closed tip of thetapered duct a transverse V-shaped groove at a machining angle in therange from 25 to 50°, such as 35 to 45°. The angle of the groove has animpact on the shape of the oval flow opening thus produced and hence onthe shape of the jet produced. The nozzle of the invention provides afairly rounded fan-shaped jet with soft edges, thus facilitatingoverlapping of adjacent jets so as to achieve optimally regular coating.

The flow duct is preferably circular in cross-section and straight.Before machining, the duct tip has preferably the shape of a sphericalsurface.

Enlargement of the V-shaped groove has proved to increase the wearresistance of the nozzle. In high-pressure spraying, flow rates are high(e.g. on the order of about 100 m/s), and coating compositions usuallycomprise solid substances (e.g. calcium carbonate), which substantiallyincrease the wear of nozzles.

The nozzle may comprise a preliminary nozzle. It acts as a preliminarydiffuser of the jet. The preliminary nozzle may especially comprise anexpanding flow channel. It is particularly useful for enhancing the wearresistance of the nozzle. In a number of embodiments, the flow channelof the preliminary nozzle may expand or taper in the flow direction.

The size (diameter of orifice) of the preliminary nozzle may be e.g. inthe range from 0.1 to 1 mm, typically in the range from 0.25 to 0.55 mm.The area of the preliminary nozzle orifice may account for e.g. at themost 50%, typically at the most 20% of the orifice area of the nozzleproper (secondary nozzle).

Also, a nozzle has now been invented, in which the ratio of the maximumdiameter to the minimum diameter of the oval orifice is markedly morethan 1, such as 1.2 to 3, especially 1.5 to 2.5. The nozzle orifice mayhave dimensions e.g. in the range from 1 to 0.3 mm×0.5 to 0.1 mm,typically 0.75 to 0.4 mm×0.35 to 0.15 mm.

Also, a nozzle has now been invented that comprises a secondary nozzle,a tapered flow duct and a preliminary nozzle connected in front of this,the area of the flow opening of the preliminary nozzle being at the most1.1 times the transverse area of the flow opening of the secondarynozzle. Optimally, the area of the flow opening of the preliminarynozzle is at the most equal to the transverse area of the flow openingof the secondary nozzle. Such a preliminary nozzle allows for increasedwear resistance of the preliminary nozzle.

The nozzles of the invention can be used in the coating of paper, suchas printing paper and cardboard, for instance.

Some embodiments of the invention are described in detail below. Theaccompanying drawings pertain to the written description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a nozzle of the invention and a preliminary nozzle to beconnected to the nozzle.

FIG. 2 shows the volume flow of the nozzle combination in FIG. 1 as afunction of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The nozzle of FIG. 1 comprises a secondary nozzle 1 and a preliminarynozzle 2.

The secondary nozzle 1 has been manufactured by first making a piecehaving a straight tapered flow duct, which is circular in cross-sectionand comprises a closed tip shaped as a spherical surface. In the centerof the tip, a transverse V-shaped groove has been machined so as toprovide a nozzle orifice 3 with the desired transverse area. The nozzleorifice 3 is oval and it produces a fan-shaped jet.

The preliminary nozzle 2 comprises an expanding flow duct, whose feedorifice 4 is circular.

The grinding angle of the nozzle orifice 3 influences the shape of thenozzle orifice and the jet obtained with this. The smaller the grindingangle, the flatter the shape and the sharper the edge of the fan-shapedjet produced. The fan edges may further comprise forwardly orientedprofile peaks. An enlarged grinding angle will expand the oval shape ofthe flow cross-section, thus providing a jet profile which is rounderand better fitting with the profile of another jet.

In accordance with the invention, the grinding angle is in the rangefrom 25 to 50°, such as 35 to 45°. Accordingly, the ratio of the majoraxis to the minor axis in the oval orifice is in the range from 1.2 to3, such as 1.5 to 2.5. The fan angle of the jet thus produced is about90°. The angle between ground surface and the surface of the flow ductis preferably at least 90°, typically from 100 to 150°.

At a coating station, there may be nozzles aligned in one single row ate.g. 60 mm intervals at a distance of about 100 mm from the web. Thenozzles are preferably disposed overlapping at a suitable angle with aview to providing optimally regular double coverage.

It has also been found that the corner of the lower edge 5 of thegrinding side is most critical in terms of wear. This corner is roundedduring the wear of the nozzle, resulting both in a larger orifice areaand altered orifice geometry and consequently also in a different jetshape. The originally oval orifice will approach a rectangular shape.The larger the grinding angle, the lower the abrasion.

The impact of abrasion was studied with regard to a nozzle of FIG. 1 byspraying calcium carbonate paste (50% dry matter content) under apressure of 10 MPa. The volume flow (ml/s) as a function of time (h) isindicated in FIG. 2. The volume flow increases very strongly at theoutset. However, at the end of about 95 hours, the growing rate isdistinctly stabilized. At 336 hours, the preliminary nozzle wasreplaced, resulting in a 32% drop in the volume flow, which still was34% higher than the starting level. Subsequently, the abrasion curvewill be slightly gentler than that of two new nozzles. This ispresumably due to the fact that a new preliminary nozzle has a smallerorifice than that of a worn secondary nozzle. As a preliminary nozzlehas larger area of wear, the secondary nozzle will wear at a slowerrate. As the abrasion curve stabilizes, the sizes of the nozzle orificeareas approach each other. As the secondary nozzle was replaced at 670hours, the volume flow started to grow strongly again, thus supportingthe assumption above.

When a preliminary nozzle of one size category below was fitted in thenozzle, abrasion became markedly slower. Over two weeks (336 h), thevolume flow increased by 25% alone, and this can be readily compensatedfor with the aid of pumping pressure.

The area of the flow orifice of a preliminary nozzle should not be morethan 1.1 times the transverse area of the flow orifice of the secondarynozzle. The area of the flow orifice of the preliminary nozzle ispreferably at the most equal to the transverse area of the flow orificeof the secondary nozzle.

1. An array of nozzles for use in coating by high-pressure sprayingtechniques a web of material moving in a first direction, the array ofnozzles comprising: at least one row of a plurality of secondary spraynozzles oriented transverse to the first direction; wherein eachsecondary spray nozzle has portions defining a tapered duct, tapering toa closed tip in which a V-shaped groove has been machined, the V-shapedgroove defining a secondary nozzle orifice defining a transverse nozzleorifice area, the secondary nozzle orifice arranged to form a jet ofcoating material; and wherein the V-shaped groove has a first side and asecond side which intersect to define an angle which is between 25 to 50degrees; wherein each secondary spray nozzle is connected to apreliminary nozzle, and wherein the preliminary nozzle starts from apreliminary nozzle orifice which leads into an expanding duct, theexpanding duct being connected to the tapered duct of the secondaryspray nozzle so that a flow into the preliminary nozzle orifice leadsinto the expanding duct of the preliminary nozzle and then into thetapered duct of the secondary spray nozzle and then to the secondarynozzle orifice.
 2. The array of nozzles of claim 1, in which the angleis between 35 to 45 degrees.
 3. The array of nozzles of claim 1, whereineach secondary transverse nozzle orifice is oval in shape.
 4. The arrayof nozzles of claim 1, wherein the secondary nozzle orifice defines amaximum diameter and a minimum diameter and a ratio between said maximumdiameter and said minimum diameter which is greater than 1.2.
 5. Thearray of nozzles of claim 4, wherein the ratio is between 1.2 and
 3. 6.The array of nozzles of claim 5, wherein the ratio is between 1.5 and2.5.
 7. The array of nozzles of claim 1, wherein the secondary nozzleorifice has dimensions of between 1.0-0.3 mm by between 0.5-0.1 mm. 8.The array of nozzles of claim 7, wherein the secondary nozzle orificehas dimensions of between 0.75-0.4 mm by between 0.35-0.15 mm.
 9. Thearray of nozzles of claim 1, wherein each preliminary nozzle orificedefines an orifice area which is at the most 1.1 times the transversesecondary nozzle orifice area to which the preliminary orifice isconnected.
 10. The array of nozzles of claim 9, wherein the orifice areaof the preliminary nozzle orifice is at most equal to the secondarytransverse nozzle orifice area.
 11. The array of nozzles of claim 1,wherein the preliminary nozzle orifice has a diameter of between 0.1 mmand 1 mm.
 12. The array of nozzles of claim 11, wherein the diameter ofthe preliminary nozzle orifice is between 0.25 and 0.55 mm.
 13. Thearray of nozzles of claim 1, wherein the preliminary nozzle orifice hasan area which is equal to or less than 50 percent of the secondarytransverse nozzle orifice area.
 14. The array of nozzles of claim 13,wherein each preliminary nozzle orifice has an area which is equal to orless than 20 percent of the secondary transverse nozzle orifice area.15. The array of nozzles of claim 1 wherein the web of material is apaper web.
 16. An array of nozzles with reduced wear characteristics foruse in coating by high-pressure spraying techniques a moving web ofpaper or cardboard mounted for motion in a first direction, the array ofnozzles comprising: at least one row of a plurality of secondary spraynozzles which are arrayed transverse to the first direction; whereineach secondary spray nozzle has portions defining a tapered duct whichtapers to a tip which is closed but for portions of the nozzle forming atransverse V-shaped groove which intersects the tip, thereby defining asecondary nozzle orifice which defines a transverse nozzle orifice area,the secondary nozzle orifice arranged to form a jet of coating material;and wherein the V-shaped groove has a first side and a second side whichintersect to define an angle which is between 25 to 50 degrees; whereineach secondary spray nozzle is connected to a preliminary nozzle, sothat the tapered duct of the secondary spray nozzle is in flow receivingrelation to portions of the preliminary nozzle forming an expanding ductwhich starts from and is in flow receiving relation to portions of thepreliminary nozzle forming a flow orifice, the flow orifice defining anarea, the flow orifice in flow receiving relation to a supply of coatingat a pressure between 1 MPa and 200 Mpa.
 17. The array of nozzles ofclaim 16, wherein the secondary nozzle orifice defines a maximumdiameter and a minimum diameter and a ratio between said maximumdiameter and said minimum diameter which is greater than 1.2.
 18. Thearray of nozzles of claim 16 wherein the preliminary nozzle flow orificearea is at the most 1.1 times the transverse area of the secondarynozzle orifice of the connected secondary spray nozzle.
 19. The array ofnozzles of claim 16 wherein the flow orifice of the preliminary nozzlehas a diameter of between 0.1 mm and 1 mm.
 20. The array of nozzles ofclaim 16 wherein the area of the flow orifice of the preliminary nozzleis equal to or less than 50 percent of the transverse area of thesecondary nozzle orifice of the connected secondary nozzle.